1. Field of the Invention
The present invention relates to an optical scanner that scans a light beam, and particularly to an optical scanner employed in the scanning device of an electrophotographic image-forming device.
2. Description of Related Art
Electrophotographic image-forming devices, such as laser printers, employ an optical scanner (scanning device) to emit a beam of light onto a photosensitive drum, enabling the exposed regions of the drum to carry a toner image. The toner image carried on the photosensitive drum is subsequently transferred onto a recording medium, such as paper or a transparency, forming an image thereon.
The optical scanner is configured of a light source, a rotating polygon mirror, and imaging means, including mirrors and lenses. Normally, at least the polygon mirror and the imaging means are accommodated in a casing for protection from dust.
Since the optical scanner emits light beams for exposing photosensitive drums in the electrophotographic image-forming device, exit holes are formed in the casing of the optical scanner to allow passage of the light beams. Image-forming devices, such as that disclosed in Japanese unexamined patent application publication No. HEI-6-194588, are provided with transparent covers formed of glass or the like (hereinafter referred to as an exit hole covers) to seal the exit holes, thereby protecting the polygon mirror, imaging means, and the like from dust particles.
However, since the image-forming devices disclosed in Japanese unexamined patent application publication No. HEI-6-194588 deflects and scans light beams emitted from the light sources by rotating the polygon mirror, the rotating motion of the polygon mirror agitates air in the casing, producing airflow. Consequently, air outside the casing is drawn into the casing through gaps between the exit hole cover and casing.
Of the gaps between the exit hole cover and casing in fluid communication with the region outside the casing, some gaps are too small to allow the passage of dust particles. These gaps act as a filter and trap and accumulate dust particles therein.
At this time, if the exit hole cover 436 is disposed on the inside of the casing 430 with the exit hole 435 exposed on the outside of the casing, as shown in FIG. 11A, dust particles that cannot pass through gaps between the exit hole cover 436 and the casing 430 accumulate in edges of the exit hole 435, obstructing the same.
In other words, the construction employed in Japanese unexamined patent application publication No. HEI-6-194588 is problematic in that dust particles tend to accumulate on the exit hole covers, obstructing the exit holes.